Hyperspectral imaging collects and processes information from across the electromagnetic spectrum. Hyperspectral imaging may utilize light in the electromagnetic spectrum ranging from ultraviolet to infrared light. Hyperspectral capabilities enable the recognition of different types of organisms, all which may appear as the same color to the human eye. Hyperspectral sensors differentiate objects based upon unique “fingerprints” across the electromagnetic spectrum that are known as spectral signatures and enable identification of the materials that make up a scanned object. Hyperspectral sensors collect information as a set of “images” with each image representing a range of the electromagnetic spectrum, also known as a spectral band. Such “images” may be combined to form a three dimensional hyperspectral cube for processing and analysis.
Spectroscopic imagers have been developed for a variety of biomedical applications, from retinal oximeters (see W. R. Johnson, D. W. Wilson, W. Fink, M. Humayun, and G. Bearman, “Snapshot hyperspectral imaging in opthalmology,” J. Biomed. Opt., 12, 14036-14043, (2007) and J. C. Ramella-Roman, S. A. Mathews, “Spectroscopic Measurements of Oxygen Saturation in the Retina,” (IEEE J. of Selected Topics in Quantum Electronics 13, 1697-1703, 2007) to evaluation of skin burn depths (see M. Soya, L. Leonardi, J. Payette, J. Fish, H. Mantsch, “Near Infrared spectroscopic assessment of hemodynamic changes in the early post-burn period,” Burns 27, 241-249 (2001) and evaluation of skin lesions (see, e.g., M. Hassan, R. Little, A. Vogel, K. Aleman, K. Wyvill, R. Yarchoan, and A. Gandjbakhche, “Quantitative assessment of tumor vasculature and response to therapy in kaposi's sarcoma using functional noninvasive imaging,” Technol. Cancer Res. Treat. 3(5), 451-457 (2004)).
Depending on the application, spectroscopic imagers are completely passive (as disclosed in W. R. Johnson, D. W. Wilson, W. Fink, M. Humayun, and G. Bearman, “Snapshot hyperspectral imaging in opthalmology,” J. Biomed. Opt., 12, 14036-14043, (2007) and J. C. Ramella-Roman, S. A. Mathews, “Spectroscopic Measurements of Oxygen Saturation in the Retina,” (IEEE J. of Selected Topics in Quantum Electronics 13, 1697-1703, 2007) or are able to switch through different wavelengths by tuning a wavelength dependent apparatus, as in the case for Liquid Crystals Tunable Filters (LCTF) and Acoustic Optics Tunable Filters (AOTF). Compact hyperspectral imagers based on AOTF have been developed at the Army Research Laboratory. Reports on the same are in publications N. Gupta, R. Dahmani, and K. Bennett, S. Simizu, D. R. Suhre, and N. B. Singh, “Progress in AOTF Hyperspectral Imagers,” in Automated Geo-Spatial Image and Data Exploitation, W. E. Roper and M. K. Hamilton, Eds., Proc. SPIE 4054, 30-38, (2000); N. Gupta, L. Denes, M. Gottlieb, D. Suhre, B. Kaminsky, and P. Metes, “Object detection using a fieldportable spectropolarimetric imager,” App. Opt. 40, 6626-6632 (2001); N. Gupta, R. Dahmani, and S. Choy, “Acousto-optic tunable filter based visible-to near-infrared spectropolarimetric imager,” Opt. Eng. 41, 1033-1038 (2002); 8. N. Gupta, and V. Voloshinov, “Hyperspectral Imager from Ultraviolet to Visible Using KDP AOTF,” Appl. Opt. 43, 2752-2759 (2004); N. Gupta, “Acousto-optic tunable filters for Infrared Imaging,” Proc SPIE 5953, 59530O 1-10 (2005); N. Gupta, “Acousto-Optic Tunable Filter-based Spectropolarimetric Imagers for Medical Diagnostic Applications—Instrument Design Point of View,” Journal of Biomedical Optics (JBO), 10, 051802-1-6 (2005); N. Gupta and D. R. Suhre, “AOTF imaging spectrometer with full Stokes polarimetric capability,” Appl. Opt. 46, 2632-2037 (2007).
A number of hyperspectral imagers were built covering different spectral regions from the ultraviolet (UV) to the longwave infrared (LWIR). Such imagers can collect data at the wavelengths of interest, which is critical for hyperspectral applications because it greatly reduces the data processing requirements associated with traditional hyperspectral imaging systems using gratings and prisms where images are acquired in hundreds of bands without much flexibility. Optical tunable filter (OTF) imagers can switch among wavelengths in tens of micro-seconds, much faster than liquid crystal tunable filters (LCTF) that have 50 to 500 ms operating time. Unlike a traditional grating, prism or LCTF an acousto-optic tunable filter (AOTF) is also a polarization sensitive device because the diffracted beams from it are orthogonally polarized. By combining the AOTF with a spectrally tunable retarder to change the polarization of incident light on the imaging system, polarization information from the scene or subject of interest can also be obtained.
Portable Acousto-optical Spectrometers are disclosed in U.S. application Ser. No. 11/208,123, filed Aug. 18, 2005, which issued on May 19, 2009, as U.S. Pat. No. 7,535,617 to Gupta, et al, which is hereby incorporated by reference as though fully rewritten herein. As disclosed in U.S. Pat. No. 7,535,617, the AOTF is a birefringent crystal having an acoustic transducer bonded to one face. Broad-band light radiation passing through a crystal can be diffracted into specific wavelengths by application of a radio-frequency (rf) driving signal to the crystal transducer. Among the attractive features of AOTFs are their small size, light-weight, computer-controlled operation, large wavelength tuning range, and reasonably high spectral resolution. Additionally, their operation can be made ultra-sensitive by using advanced signal-processing algorithm.
A number of different crystals, i.e., quartz, LiNbO3, etc., allow collinear diffraction of light with either longitudinal or shear acoustic wave propagation. Chang generalized the design of an AOTF cell by introducing the concept of a noncollinear AOTF using tellurium dioxide (TeO2), a birefringent crystal (a crystal having two refractive indices) that cannot exhibit collinear interaction because of its crystal symmetry. In a noncollinear AOTF cell the incident light, the diffracted light, and the acoustic wave do not travel in the same direction.
An AOTF is essentially a real-time programmable filter whose operation can be described as follows. When white light is incident on the filter, it passes only a selected number of narrow bands corresponding to the applied rf-signals. The filter can be used to pass light with either a single wavelength or multiple wavelengths, depending upon the number of applied rf-signals. Either a collinear or a non-collinear geometry can be used in designing an AOTF cell, based on the symmetry properties of the anisotropic crystal under consideration. The incident light is linearly polarized by a polarizer in front of the crystal before it enters the AOTF cell. As this polarized light passes through the cell, it is diffracted in the same direction by a diffraction grating set up by the collinearly traveling sound wave. Owing to conservation of energy, the frequency of the diffracted light is Doppler shifted, but this frequency shift is insignificant and can be ignored. Based on conservation of momentum, a tuning relationship can establish between the center wavelength of the filter and the applied rf-signal. Many excellent review articles on AOTF technology and applications are available, for example see Gottlieb, M. S., “Acousto-optic tunable filter,” Design and Fabrication of Acousto-Optic Devices, A. P. Goutzoulis and D. R. Pape, eds., Marcel Dekker, New York, 1994, pp. 197-283; Gupta, N., ed., Proceedings of the First Army Research Laboratory Acousto-Optic Tunable Filter Workshop, Army Research Laboratory, ARL-SR-54 (1997); and Gupta, N. and Fell, N. F., Jr., “A compact collinear Raman spectrometer,” Talanta 45, 279-284 (1997). A more complete description is found at N. Gupta, “Biosensors Technologies-Acousto-Optic Tunable Filter based Hyperspectral and Polarization Imagers for Fluorescence and Spectroscopic Imaging,” in “Methods in Biotechnology,” edited by Avraham Rasooly and Keith E. Herold by the Humana Press Inc., Totowa, N.J., page 293-305, (November 2008).
An example of a spectrometer using AO crystal cells is found in U.S. Pat. No. 5,120,961 entitled “High sensitivity acousto-optic tunable filter spectrometer,” hereby incorporated by reference, which teaches of using an acousto-optical filter (AOTF) device in a spectrometer. This spectrometer operates by using continuous wave RF-excitation through the crystal, wherein the spectrometer provides control and modulation of the RF-source. Noise is minimized by a lock-in amplifier that demodulates the modulation frequency. Fiber optics are used to connect the crystal to the source, and the source to the detection system.
One AOTF-based imager operates from the visible to the near infrared (400-800 nm). See N. Gupta, R. Dahmani, and S. Choy, “Acousto-optic tunable filter based visible-to near-infrared spectropolarimetric imager,” Opt. Eng. 41, 1033-1038 (2002), hereby incorporated by reference. This imager operates in a passive mode by detecting the light either reflected or transmitted by an object. By using an electronically tunable liquid crystal variable retarder (LCVR) as a function of wavelength in the path of the incident light on the AOTF, the imagers are shown to detect both spectral and polarization signatures. In the article, a compact, lightweight, robust, and field-portable spectropolarimetric imager is developed to acquire spectropolarimetric images both in the laboratory and outdoors. The described imager used a tellurium dioxide (TeO2) acousto-optic tunable filter (AOTF) as an agile spectral selection element and a nematic liquid-crystal variable retardation (LCVR) plate as a tunable polarization selection device with an off-the-shelf chargecoupled device (CCD) camera and optics. The spectral range of operation was from 400 to 800 nm with a 10-nm spectral resolution at 600 nm. Each spectral image was acquired with two retardation values corresponding to the horizontal and vertical incident polarizations. The operation of the imager and image acquisition was computer controlled. For a further description of the instrument and its operation and present results of measurements, see the N. Gupta, et al., “Acousto-optic tunable filter based visible-to near-infrared spectropolarimetric imager,” Opt. Eng. 41, 1033-1038 (2002), hereby incorporated by reference.
Turning to the medical field, currently an estimate of the oxygen saturation in the blood of a human body can be made with a clip that fits on the subject's finger. The clip operates by shining a light through the subject finger; and a detector measures the light that comes through the other side. The machine functions on the basis that oxygen saturated blood cells absorb and reflect light differently than those that are not. Blood cells are a bright red when they are loaded with oxygen, and they change to a bluish color when they are no longer carrying a full load. Such machines give only a rough estimate a body's oxygen saturation and its measurement can be affected such things as red nail polish on the finger. A more accurate test for measuring oxygen saturation of the blood is an arterial blood gas test; commonly obtained using a blood sample, however, such tests require the availability of the subject's blood and time for the analysis.
The measurement of the oxygen deficiency in the blood is an indicator of hypoxia oxygen deficiency, which occurs when there is an inadequate supply of oxygen to tissue. An inadequate supply of oxygen to tissue may be the result of a variety of factors, including an impairment or reduction in partial pressure of oxygen, inadequate oxygen transport, or the inability of the tissues to use oxygen. Reduction of the oxygen carrying capacity of the blood (or adequately oxygenated blood) due to circulation, liver, or heart disorders, causes tissue death. Conversely, oxygen deficiency in the body tissue is an indicator for disease, poisoning, and resulting death of tissue. Brain cells are extremely sensitive to oxygen deficiency and can begin to die within five minutes. Causative factors such as drowning, strangling, choking, suffocation, cardiac arrest, head trauma, and carbon monoxide poisoning can create conditions leading to cerebral hypoxia, which can lead to coma, seizures, and even brain death. Similarly, carbon monoxide and cyanide poisoning may lead to histotoxic hypoxia, which is the inability of body tissues to use oxygen. Also, certain narcotics will prevent oxygen use by the tissues. Conversely, lack of the presence of oxygen in body tissue may be indicative of poisoning, chemicals, or certain narcotic usage. Hypoxia may lead to a complete absence of oxygen in tissue or anoxia; a condition where the metabolism of cells is disrupted causing tissue cells to die within minutes.
In situations where common diagnostic procedures are not available or inadvisable to determine the medical condition of a human body, remote diagnosis (which does not involve human contact or contamination) based upon oxygen deficiency may be advantageous. Accordingly, there exists a need to determine blood oxygen content in body tissue without exposing others to potential diseases, biological agents, radiation hazards, or the causative factors of the oxygen deficiency. Since death may result within minutes of an extreme oxygen deficiency, a quick response time or diagnosis is not only highly desirable, but may be imperative.